1. Field of the Invention
The present invention generally relates to analog to digital (A/D) converters and, in particular, to such converters which use a charge balance conversion technique.
2. Statement of the Prior Art
Charge balance analog to digital converters were initially developed in the early 1960's as a modification of the dual slope technique and as evidenced by U.S. Pat. Nos. 3,267,458 and 3,281,827. Generally, an unknown input signal and a reference signal are both coupled to the input of a single integrator which has its output signal compared to ground. While the input signal is constantly applied to the integrator input, the reference signal is alternately coupled to and uncoupled from the integrator input, which coupling changes the direction of change of the integrator output signal. The uncoupling is determined by the comparator indicating that the integrator output has crossed zero volts. The reference signal is then recoupled a fixed time period after the uncoupling. This process reoccurs a multiplicity of times over a predetermined period and the number of times or amount of time that the reference signal is coupled is measured, usually by a digital counter or clock. The time of coupling is then proportional to the unknown input signal.
The current state of the art may be evidenced by an A/D converter chip set sold by Siliconix and designated LD110/LD111A. Control logic coupled to the comparator outPut determines the duty cycle of the reference signal coupling. One of two duty cycles is selected depending upon whether the comparator output is either positive or negative, to cause the integrator output to tend towards zero volts.
As with many analog to digital converters, there are a number of cost and performance trade offs which may be made within the construction of a converter to optimize its performance for any given application. SPecific performance parameters include conversion speed, resolution and accuracy. An example of a performance trade off would be speed versus resolution. A cost trade off would include the cost of precision components to produce a certain level of accuracy at a given speed and resolution.
With these variable parameters in mind, the measure of advances in the art of conversIon is concerned with not just the specific parameters but also with improving all of the performance parameters at once so that component costs may be simultaneously reduced without sacrificing performance.